session expired -> how to know if the session is new or old

Hi, Here is the problem: The session gc runs and deletes the session data. This is good, but calling session_start in PHP obviously creates a new session. How can I know if the session was created after an old one was deleted because gc has invalidated it or it is a completely fresh session, from let's say a fresh browser window. I googled for it and looked in the docs but I found nothing on this matter. Thanks in advance, Catalin